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William Gill 25a2182c45 v1.29-notify-3: rewrite daemon in Go, fix record size

Two changes from v1.29-notify-2:

1. kmod: fix a latent ring-size compile error.  The record struct was
   56 bytes with an _pad[6] trailer, which meant
   NOTIFY_RING_BYTES / sizeof(record) = 1170 events per ring —
   not a power of two, so the BUILD_BUG_ON in notify_setup would
   have fired.  Bumped _pad to 14 bytes so the record is 64 bytes,
   capacity is 1024, and every record is cache-line aligned.  Wire
   ABI bumped accordingly; no deployed consumers yet.

2. scoutfs-notifyd: rewritten in Go using the standard library
   only.  Three goroutines (reader / accept / broadcaster) with
   channel communication replace the single-thread epoll + ioctl
   loop from the C version.  Slow-client handling is done by
   setting an in-the-past write deadline; the broadcaster drops
   clients whose kernel send buffer is full.

   Packaging changes: utils/Makefile now invokes "go build" with
   CGO_ENABLED=0 and GOPROXY=off; utils/scoutfs-utils.spec.in adds
   BuildRequires: golang >= 1.21.

2026-04-22 14:34:12 -05:00

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scoutfs-notify

Observer-only file access notifications for ScoutFS, distributed as a rebasable git format-patch series plus a small Go userspace relay daemon. Layers onto each upstream release with minimal maintenance.

What the series adds

Three patches against the scoutfs source tree:

Kmod core — a per-mount 64 KiB ring of 64-byte notification records and a single-reader drain ioctl (SCOUTFS_IOC_READ_NOTIFY, nr 25). Emit is non-blocking, drop-on-full; the monotonic seq field exposes drops to consumers. Three percpu counters (notify_emitted, notify_dropped_ring_full, notify_reader_attached). No mount option, no sysfs toggle.
Kmod hooks — ->open wrapper and READ emit in scoutfs_file_aio_read / scoutfs_file_read_iter. Every hook is a single predicted-false branch when no reader is attached. Nothing in the data-waiter state machine is touched.
scoutfs-notifyd (Go) — userspace daemon that binds /run/scoutfs/<fsid>/notify.sock (AF_UNIX SOCK_SEQPACKET, mode 0600, root-only), drains the ring, and broadcasts each record to connected clients. Pure-stdlib Go; no external module dependencies. Shipped with a systemd template unit scoutfs-notifyd@<mountpoint>.service.

Base

Currently rebased against:

scoutfs v1.29

See base.txt.

Build dependencies added

On top of the stock scoutfs build requirements, patch 3 adds:

golang >= 1.21 on the build host (RHEL/EL9: golang, EL8: go-toolset). Debian 12 / Ubuntu 22.04+: golang-go.

The Go build is offline (GOPROXY=off) — no network access required at build time. CGO_ENABLED=0 so the produced binary is a pure-Go static ELF.

Applying

./apply.sh /path/to/scoutfs

Runs git am --3way on each patch. For a tarball instead of a git tree, loop patch -p1 < patches/*.patch.

Rebasing onto a new upstream release

git fetch --tags
git checkout -B notify v1.30
git am --3way patches/*.patch
# resolve any conflicts, git am --continue
git format-patch v1.30..notify -o patches/
# update base.txt and commit

Tag history

v1.29-notify-3   (current — Go daemon, struct alignment fix)
v1.29-notify-2   (retired — C daemon; had latent ring-size compile bug)
v1.29-notify-1   (retired — shipped mount options)

Quick smoke test after installation

systemctl enable --now scoutfs-notifyd@mnt-scoutfs.service
FSID=$(stat -f -c %i /mnt/scoutfs)   # or use your scoutfs cli
socat - UNIX-CONNECT:/run/scoutfs/${FSID}/notify.sock | xxd | head
# touch some files in /mnt/scoutfs in another terminal

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